1. Field of the Invention
This invention relates to a method and apparatus for providing a directory number to a call-processing device in a communications network
2. Descrption of the Prior Art
A "complete directory number" is a series of one or more digits, such as a telephone number, that is associated with a particular customer identity in a communications network. Usually a complete directory number relates to a particular terminal within the communications network. For example, the complete directory number could be a telephone number associated with a particular mobile telephone terminal or with a particular fax machine. Also, the complete directory number could be a telephone number associated with terminals of an automatic call distribution system.
The term "directory number" is used here to refer to directory numbers that may be complete or may be incomplete. An incomplete directory number is one where some of the digits are missing, e.g. because the user has not yet dialled them.
Historically, complete directory numbers (CDNs) have been allocated by service providers according to numbering plans devised by the service providers themselves. For example, some service providers allocate CDNs according to an "open dial plan". An "open dial plan" is a scheme for allocating directory numbers to customers which allows the length of the customer's CDN (i.e. the number of digits in the CDN) to vary from customer to customer. On the other hand, other service providers allocate CDNs according to a "closed dial plan" and every customer's CDN is the same length.
Open dial plans are complex because variations in the length of nearly all parts of the CDN often occur. For example, the regional code might be one digit for a particular geographical area (such as "1" for London) and for other regions may be three or four digits. For metropolitan areas, many digits are required in addition to the regional code because of the number of customers in the area. For example, if a regional code of one digit is used for London, then a director system of 7 digits (XXX-XXXX) might be required. At other, perhaps rural, exchanges there might be only a few hundred telephone lines so that only 3 digits are required.
In an open dial plan, if the complete directory number 123 exists, then the complete directory number 1234 cannot normally exist. If it did then calls to directory number 1234 would never reach that number because they would be recognised as being for 123 after the third digit.
Closed dial plans are more simple than open dial plans in that every CDN is the same length. This makes it easier for users and equipment to determine when there is an error in a telephone number that they have been given, for example, because it is not long enough. However, both open and closed dial plans are complicated when a localised region grows faster than predicted and more digits are needed to serve customers who require the communications service.
Mixed dial plans are effectively open dial plans that have become too complex to maintain economically at digit collectors.
The existence of closed dial plans (prevalent in North America) and open dial plans (often found in Europe) has led to the development of two distinct schemes for routing a call over a digital telephone network. These schemes typically align with the type of dial plan found in an equipment manufacturer's home market.
The scheme for routing a call in a digital network based on a closed dial plan is now described. Since every CDN is the same length, when a customer starts to dial, the user's switch in the telephone network collects the expected number of digits before attempting to route the call. The dialled digits are divided according to a simple rule into those needed to identify the area (the area code) and the number within that area. For example, consider a North American number 1-12345-67890. As dialled this number would be collected at a switch termed the "originating switch" . All the digits are collected and then the first digits are analysed. The leading "1" identifies a long-distance number and therefore the next three digits are used to decide where to send the last 7 digits. On this basis the last 7 digits are sent to an exchange near to the destination address. This local exchange interprets the remaining digits and the call is completed. Some areas may use the first 3 of the last 7 digits to identify an exchange which subtends 10,000 lines that are addressed by the final 4 digits. However, this distinction is often lost where large modem switches which operate for many more than 10,000 lines are used. Such modem switches actually accommodate many of the "old" switches which only survive as partitions in the larger switch.
A method of routing a call in a network based on an open dial plan is now described. First the target area is identified from the leading digits and then every subsequent digit is passed over the network to the target area one at a time. Every subsequent digit has to be passed in this way because the exact number of digits in the CDN is not known before the user finishes dialling. The subsequently dialled digits are used to identify further switching points and the dialled digits are passed on to the next point of ambiguity. This means that many more signalling messages have to be passed between switches before the call is completed than is the case for a closed dial plan system. This is a major disadvantage for open dial plan systems.
A problem arises when systems which have been designed to work with closed dial plans are used in markets or applications with open dial plans. The closed dial system waits to receive a fixed number of digits before doing anything else and so, unless the CDN is coincidentally the expected length, a problem occurs. If the CDN is shorter than expected, then the switch typically waits for further digits until a timeout (an inter-digit time out) expires. At this stage, the switch attempts to complete the call with the digits that have been collected. This succeeds because all the digits have been collected, but the user observes a "dead time" while the time out expires. The "time out" is typically 6 seconds long and this length of delay quickly becomes irritating for the customer and makes the system disadvantageous. If shorter "time out" delays are used other problems are caused. For example, the call may be attempted while the user is still entering digits. Another problem is that if the CDN is longer than expected then the call is not completed. This is because the call will be attempted with too few digits and subsequently dialled digits are ignored. In order to avoid this problem the "expected" CDN length is set to the maximum possible CDN length in the open plan system. This maximum can be very long and because of this the user nearly always experiences the inconvenience of a post-dial delay.
A similar problem is encountered when older technologies are integrated with newer technologies, such as a GSM mobile network. For example, when a customer originates a call from a mobile telephone, such as a GSM phone, the customer keys in the whole DN and then keys a "send" button. The whole DN is then sent to the GSM switch (known as an MSC) which then attempts the call on the assumption that the DN is complete. If the DN is incomplete the call attempt fails and the user is informed. For an open dial plan system, this means that valuable processing capacity has been taken up attempting to make a call for which the DN has been subsequently proven to be incomplete. If special software is written to enable fixed access systems to be integrated with a GSM switch there are several disadvantages, including processing time, memory constraints, development costs, maintenance and testing.
If traditional fixed accesses (traditional telephone systems) are used with a mobile network switch (usually used for mobile telephones), for example, as part of a fixed-mobile convergence programme, then the user does not have a "send" key to indicate that the DN is complete. In this case, the access network or the mobile switch has to determine when the DN is complete by some other means before attempting the call. However, it is difficult to provide "other means" by which this is determined.
Three approaches have previously been taken to address the problem of post-dial delay. The first approach involves re-writing closed dial plan switch software so that it will be able to cope with open dial plan systems. This re-writing is a complex and lengthy process because the original switch software is based on a closed dial plan system and so "digit capture" is at the core of the software and cannot be easily adapted. By carrying out this re-write the software is arranged to pass each digit on to the next switch in the connection one at a time. Also, the switch is modified so that it does not attempt to accumulate digits beyond the number necessary to identify the next switching point. There are a number of problems with this approach. The signalling load is increased significantly because of the need to pass the additional single-digit messages through the network. Also, it is expensive, complex and lengthy to re-write the switch software as required. Furthermore, this method cannot easily and economically be used to address the integration of fixed access systems to a mobile network.
A second approach has been to require the customer to press a special key after the DN has been dialled (e.g. a "send" key). This abbreviates the post-dial delay. For example the "#" key is often used for this purpose but it is unpopular and inconvenient for the user. Also, simple telephones which use loop-disconnect (pulse) dialling instead of tone (dual tone multi-frequency, DTMF) dialling remain incompatible.
A third approach is to use initial digits to identify that the DN belongs to a closed dial plan. The list of pre-fixes can be very large so that many closed dial plan regions are known about. For example, the international pre-fix (e.g. 00) followed by the North American code (1) would indicate that exactly another 10 digits are required (xx-xxx-xxxx). Similarly, dialling the prefix for a mobile network (0171 in Germany, for example) would indicate that exactly another 7 digits are required (xxx-xxxx). This solution is extremely powerful, but depends upon the individual digit collectors being educated with the identifying characteristics of the closed dial plans, and being re-educated if these characteristics change.
It is accordingly an object of the present invention to provide an apparatus and method for providing a directory number to a call-processing device in a communications network which overcomes or at least mitigates one or more of the problems noted above.